This invention relates to a method and system for a non-destructive inspection of the internal pressure of hermetically sealed containers such as bottled and canned goods. More particularly, it provides a method and system whereby cans housed in a carton case can be inspected from the exterior without the need to open the case.
Heretofore, various methods of inspecting a single can being conveyed on a conveyor have been employed. Also, the possibility of inspection within a carton case is disclosed in Japanese Patent Public Disclosures Nos. 34376/74 (U.S. Pat. No. 3,802,252) and 99554/76. Furthermore, a promising method is disclosed in Japanese Utility Model Publication No. 22316/66 (U.S. Pat. No. 3,206,027).
In the first example mentioned above, vibration is generated in the cover by means of an electromagnetic pulse and, on the basis of the correlation between the number of vibrations and the internal pressure of a can, a discrimination of good or bad is made.
During a long-term storage of canned goods, sometimes the contents change in quality and evolve carbon dioxide or hydrogen gas, resulting in internal pressure increases, and the initial reduced pressure for the foods changes to a positive internal pressure.
Since the vibration of a can cover is determined by the stress of the dover surface, the relation between the internal pressure of the can and the number of vibration has a basic defect such that, as shown in FIG. 1, there is an area where the discrimination of good or bad is impossible because of the same frequency of both positive and negative pressures.
As means for detecting vibration, mention may be made of a method of measuring the damping of amplitude. In this method, however, the internal pressure of a can and the damping condition are not always correlated with each other because in a carton case the change in contacting state between cans and also between cans and the side or ceiling plate of the case greatly contributes to the damping of vibration. Thus, this method lacks utility.
The foregoing third example is a method of measuring the degree of unevenness of the can cover by the change in effective inductance of an electromagnetic coil.
A carton is an electrical insulator and this is not affected magnetically, for which reason an intra-carton measurement and discrimination is considered promising.
The above method is fairly effective for inspecting a single can on a conveyor, but the cans housed in a carton are not always arranged on the same plane. The bottom of a carton may be uneven, or as shown in FIG. 2(.alpha.)(.beta.) cans a, may contact each other resulting in the seamed portions mounting each other, thus causing tilting or floating of can. In the measurement from the exterior of the carton, the correlation with the internal pressure of a can is doubtful.
For example, in the case of a normal can 77 mm in diameter with a reduced internal pressure of 50 cmHg, a concave deflection is 1 mm at the most, while the change in distance caused by mounting of the seamed portions may be as much as 3 mm.
Because of this drawback the so-called single point measurement in which the distance to the central portion of the cover surface is measured by a fixed detector is unreliable.